1. Field of the Invention
This invention relates to a method of transmitting data and a system therefor between a plurality of electronic control units installed on an automotive vehicle via a common communication line connecting these electronic control units.
2. Prior Art
Conventionally, in performing mutual data transmission between a plurality of electronic control units (hereinafter referred to as the "ECU's" installed on an automotive vehicle via a common communication line (hereinafter referred to as "the network bus") connecting the ECU's, a token passing method is employed for circulating a transmission right round the ECU's in a predetermined sequence to permit an ECU having received the transmission right to send out data to the network bus.
When a data transmission system employing the token passing method is started, the following methods are conventionally known for initially rendering (i.e. generating) the transmission right to one of the ECU's:
(1) Fixed ECU method of initially rendering the transmission right to a particular ECU determined in advance in a fixed manner.
(2) Waiting method in which all the ECU's start transmitting data assuming that they are each initially rendered with the transmission right, stop transmitting data when collision of data occurs, and restart data transmission after waiting for predetermined waiting time periods set to the ECU's, respectively, repeatedly carrying out this procedure until no collision occurs, to thereby restrict an initial holder of the transmission right to one of the ECU's.
However, the fixed ECU method suffers from a problem that if the particular ECU to which the transmission right is to be initially rendered is in failure, the transmission right cannot be produced for ever.
With the waiting method as well, there is an inconvenience that it takes much time to generate the transmission right, and time is wastefully consumed before the ECU's start to cooperate as a network.
On the other hand, in the data transmission system using the token passing method, it is a known technique to add broadcast data to a message for transmission to transfer the transmission right to the next ECU, as disclosed e.g. in Japanese Patent Publication (Kokoku) No. 1-58900.
However, according to this conventional technique, the message data format is not constructed such that a sending end can confirm safe and accurate transfer of each of the transmission right and data, and hence there is room for improvement thereof to expedite the transmission.
Further, Japanese Patent Publication (Kokoku) No. 1-57856 (hereinafter referred to as "the first prior art") discloses a technique that when data transmission is not successfully carried out, retransmission of data is performed upon the transmission right returning to the present ECU after having been transferred round the ECU's, taking into consideration the fact that if retransmission of the data is carried out immediately, there is a high possibility of failure thereof.
Further, Japanese Provisional Patent Publication (Kokai) No. 62-159539 discloses a data transmission system (hereinafter referred to as "the second prior art") in which when transmission of the data is not successfully carried out, the data is retransmitted within a limit of a predetermined number of times, which number can be changed or set by another control system.
According to the first prior art, the retransmission of data is performed only after the transmission right has been transferred round the ECU's once even if the immediate transmission of data might be successful (e.g. when failure of transmission was caused by a noise during transmission), which is an inconvenience to be eliminated for prompt completion of data transmission.
Further, according to the second prior art, the retransmission of data is carried out immediately after failure thereof within the predetermined number of times set by the other control system. However, this number is not set by the sending end itself, and hence it is difficult to perform prompt reaction to failure of transmission.
Further, in making a check as to whether connection between one member (ECU) and another (ECU) of the network is safely established, a special message is sent out from the one member for a response from the other, based on which it is determined whether the connection is safely established.
When the special message is used, however, the data transmission and transfer of the transmission right cannot be performed while the special message is being sent out, which results in degraded transmission efficiency.
Further, when the other member (ECU) goes faulty or disconnected from the network, the state of the connection is not certain until the check therefor is carried out again, and hence there is a possibility of continuing to transfer the transmission right to the ECU disconnected.